Integrated circuits are formed on semiconductor silicon substrate wafers by a series of laying, patterning and doping steps. Patterning typically involves photolithography where a layer of photoresist is applied and then selectively removed in a masking step. This exposes those areas in the upper layer of the wafer which are to be removed or etched away.
The purpose of the etching operation is to remove portions of the top layer of the wafer through the holes, or openings, which are selectively formed in the photoresist layer. In this manner, the pattern of the photoresist mask is transferred to the top layer of the wafer. The etching operation should not disturb the underlying surface, and only remove material from the upper layer of the wafer.
Etching is typically performed with one of the following methods: wet (liquid) chemical etching, plasma etching, ion beam etching, or reactive ion etching (RIE). The term "dry etching" is a collective definition which includes any of plasma etching, ion beam etching or reactive ion etching. This disclosure concerns improved dry etching methods.
In a pure plasma etch, a high energy RF field induces gases injected into a reactor into the plasma state which causes a reaction with the exposed layer on the wafer. A pure ion beam etch, on the other hand, uses ion beams for removal of the exposed upper layer. In this method, the atoms of an inert gas (such as argon) are ionized by an RF field and directed towards the wafer. The positively charged atoms accelerate as they move toward the wafer. On impact, the top layer exposed through the photoresist is physically removed, or "blasted away" from the surface by the impinging ions.
Both the RIE method and ion beam removal method exhibit minimal undercutting beneath the photoresist. The terms "undercutting" and "isotropic etching" are synonymous and result in etching material to the side as well as in the downward direction. The term "anisotropic etching" defines an etch which removes material in only one direction. In order for an etch to be directional, there must be some directional (ion) component such as an ion beam. Plasma etch is an isotropic etch (i.e. no directionality). Pure ion beam etching promotes an anisotropic etch. However, both exhibit poor etch selectivity. "Selectivity" is a term which defines the removal of the underlying material or film. The better the selectivity for a given material, the less that material will be etched under given etch conditions. Where selectivity to the desired stop point of the etch is poor, the end point for the etch becomes critical in order to avoid overetching.
Reactive ion etching is a combination of plasma and ion beam removal at the wafer surface. The etching gas is injected into the reaction chamber and ionized. The individual molecules accelerate to the wafer surface. There, top layer removal is achieved by both physical bombardment and chemical action of the ions with the upper layer. The typical reactive ion etch employs a mixture of gases. The mixture will include one or more gases which are reactive with the surface, as well as a single inert or noble gas which provides the majority of the ion beam bombardment etching effect.
This combination bombardment/chemical etch results in an etch process that is more controllable. It also promotes improved sidewall passivation which results in an improved anisotropic etching effect. The reactive gases or etch by products form a polymer with the sidewalls of the opening as the etching proceeds down into the layer. The formation of this polymer film, termed "sidewall passivation", retards etching of the sidewall. It is removed in a later processing step after etching is completed.
As the individual components of integrated circuits get smaller, the openings in the photoresist layer for etching likewise become smaller. Reactive ion etching is credited with enabling somewhat reliable production where the opening size shrinks to about one micron wide. However, there is room for much improvement. For example, wider openings tend to etch at a different (typically higher) rate than which narrower openings are etched. This undesirable phenomenon is commonly referred to as "loading effects" due to the higher loading of components per unit area on the wafer. The goal is to achieve substantially uniform etching regardless of the size of the openings being etched, and thus that there be minimal loading effect.
In addition to uniformity of the etch regardless of opening size, it is desirable to achieve uniform etching across the surface of the wafer. Some etching gases produce a better anisotropic etching effect towards the edge of the wafer than they produce towards the center of the wafer. With such gases, the yield of usable chips at the center of the wafer is less than at the edge, thus reducing overall yield.
FIG. 1 is a diagrammatic fragmentary section illustration indicating poor center-to-edge etch uniformity in a semiconductor silicon wafer 10. FIG. 1 is a representative of an actual profile obtained of a reactive ion etch of an SiO.sub.2 layer within a small gap (0.38 cm) Tegel.TM. 903E reactor. In the context of this document, the term "small gap" refers to a parallel plate reactor where the gap between the plates is less than or equal to 1 cm. The conditions were 600 watts of applied power and a pressure of 1750 mtorr, with the upper electrode being maintained at 45.degree. C. and the lower electrode being maintained at 19.degree. C. The injected gases were SF.sub.6 at 6 sccm, CHF.sub.3 at 45 sccm, and He at 130 sccm.
Wafer 10 is represented with an outer edge 12 and a central area 14. It is comprised of a doped monocrystalline substrate 16, polysilicon sections 18a and 18b, and an SiO.sub.2 layer 20. A layer of photoresist 22 has been applied atop SiO.sub.2 layer 20, and exposed to present openings 24 and 26. These were etched by the above mixture and produced the illustrated profile. As is apparent, the center-to-edge etching uniformity is poor. Hole 24 near edge 12 exhibits an acceptable profile, while hole 26 near center 14 exhibits a poor, unacceptable profile.
FIG. 2 illustrates an etch of a wafer of the same configuration, now designated with the numeral 30, using a different gas feed but otherwise under identical conditions. Here, the injected gas mixture consisted essentially of SF.sub.6 at 6 sccm, CHF.sub.3 at 45 sccm, and Ar at 130 sccm. As illustrated, center-to-edge uniformity is good. The profile of edge opening 24a was found to be substantially the same as the center etched opening 26a profile. However, the selectivity in both instances to the polysilicon 18a, 18b is very poor, as indicated by the jagged lines. The diameter of openings 24, 24a, 26, 26a referred to in the above experimental examples were each 0.8 microns.
A need remains for an improved dry etching method having good center-to-edge uniformity and good selectivity to the underlying layer.